Answer:
Shelby cannot rescind the contract based on her mistake because she made a unilateral mistake when engaging in the contract, but Peggy is not responsible for that mistake. Shelby's mistake is considered a value mistake since she miscalculated the value of the dog.
If the mistake was a about a material fact (e.g. believing to buy one dog but actually buying another dog) then the contract could be rescinded, but if its a value mistake probably not.